Styrene acrylate emulsion (SAE) sizing agent have been widely used in the paper and paperboard industry as surface sizing agents to improve the water resistance of paper and paperboard. Depending on the monomers employed in synthesis, these SAE sizing agents can be cationic, anionic, or amphoteric, all used industrially at well-defined conditions.
These SAE sizing agents are either used by themselves or coupled with sizing aids, such as aluminum salts, and most notably aluminum sulfate (alum) and polyaluminum chloride (PAC). Once ionized in an aqueous solution, the aluminum cation helps fasten the sizing agent onto the negatively charged cellulose fibers. These sizing aids are strongly cationic and conventionally only utilized with cationic SAE sizing agents in the size press.
While polyvalent metal ions such as aluminum are effective sizing aids, the cationic nature of the aluminum ion in solution limits the papermaker and paperboard manufacturer to using only cationic dyes, cationic optical brightening agents (OBA), and other cationic additives when added to the size press. Further, cationic SAE sizing agents must also be utilized in the sizing process with the aluminum salt which results in an increase in cost for the sizing process due to the increase in cost of cationic SAE sizing agents relative to anionic SAE sizing agents. Anionic dyes, anionic OBAs, and other anionic additives will lead to the formation of coacervates with positively charged polyvalent cations, in this case aluminum from the aluminum salt. Coacervates and deposits will also form if these aluminum salts are combined with anionic SAE sizing agents.
When anionic dyes are used in the paper and board making process, typically anionic surface sizing agents, such as anionic SAE sizing agents, and solutions of styrene maleic anhydride, or styrene acrylic acid polymers are used for sizing. Aluminum salts, such as PAC and alum, cannot be used as these salts are strongly cationic. This is problematic as anionic SAE sizing agents do not bond effectively with fibers due to the negative charge on both the SAE and the fiber thereby leading to reduced sizing at equivalent dosages to the cationic system. Thus, a solution is needed for improved performance of this combination.
Accordingly, it is desirable to provide compositions and methods for utilizing SAE sizing agents, such as anionic SAE sizing agents, and aluminum salts with dyes and OBAs, such as anionic dyes and OBAs. Furthermore, other desirable features and characteristics will become apparent from the subsequent summary and detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.